Conventionally, as display elements included in a display device, there have been an electro-optical element whose luminance is controlled by a voltage applied thereto, and an electro-optical element whose luminance is controlled by a current flowing therethrough. A representative example of the electro-optical element whose luminance is controlled by a voltage applied thereto is a liquid crystal display element. On the other hand, a representative example of the electro-optical element whose luminance is controlled by a current flowing therethrough is an organic electro luminescence (EL) element. The organic EL element is also called an OLED (Organic Light-Emitting Diode). An organic EL display device using the organic EL elements which are self light-emitting type electro-optical elements can more easily achieve slimming down, a reduction in power consumption, an increase in luminance, etc., compared to a liquid crystal display device that requires a backlight, color filters, and the like. Therefore, in recent years, there has been active development of organic EL display devices.
As for driving systems for an organic EL display device, there are known a passive matrix system (also called a simple matrix system) and an active matrix system. An organic EL display device that adopts the passive matrix system is simple in structure, but is difficult to achieve an increase in size and an improvement in definition. On the other hand, an organic EL display device that adopts the active matrix system (hereinafter, referred to as “active matrix-type organic EL display device”) can more easily achieve an increase in size and an improvement in definition, compared to the organic EL display device that adopts the passive matrix system.
The active matrix-type organic EL display device has a plurality of pixel circuits formed in a matrix form. Each pixel circuit of the active matrix-type organic EL display device typically includes an input transistor that selects a pixel; and a drive transistor that controls the supply of a current to an organic EL element. Note that in the following, the current flowing through the organic EL element from the drive transistor may be referred to as “drive current.”
FIG. 37 is a circuit diagram showing a configuration of a conventional general pixel circuit 81. The pixel circuit 81 is provided at a corresponding one of intersections of a plurality of data lines DL and a plurality of scanning lines SL which are disposed in a display unit. As shown in FIG. 37, the pixel circuit 81 includes two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is an input transistor and the transistor T2 is a drive transistor.
The transistor T1 is provided between a data line DL and a gate terminal of the transistor T2. The transistor T1 has a gate terminal connected to a scanning line SL; and a source terminal connected to the data line DL. The transistor T2 is provided in series with the organic EL element OLED. The transistor T2 has a drain terminal connected to a power supply line that supplies an organic EL high-level power supply voltage ELVDD; and a source terminal connected to an anode terminal of the organic EL element OLED. Note that the power supply line that supplies an organic EL high-level power supply voltage ELVDD is hereinafter referred to as “organic EL high-level power supply line.” The organic EL high-level power supply line is denoted by the same reference character ELVDD as the organic EL high-level power supply voltage. The capacitor Cst has one end connected to the gate terminal of the transistor T2; and the other end connected to the source terminal of the transistor T2. Note that there is also a case in which the other end of the capacitor Cst is connected to the drain terminal of the transistor T2. A cathode terminal of the organic EL element OLED is connected to a power supply line that supplies an organic EL low-level power supply voltage ELVSS. Note that the power supply line that supplies an organic EL low-level power supply voltage ELVSS is hereinafter referred to as “organic EL low-level power supply line.” The organic EL low-level power supply line is denoted by the same reference character ELVSS as the organic EL low-level power supply voltage. Note also that here a connecting point of the gate terminal of the transistor T2, the one end of the capacitor Cst, and a drain terminal of the transistor T1 is referred to as “gate node” for convenience sake. The gate node is denoted by reference character VG. Note that, in general, one of a drain and a source that has a higher potential is called a drain, but in the description of this specification, one is defined as a drain and the other as a source, and thus, a source potential may be higher than a drain potential in some cases.
FIG. 38 is a timing chart for describing the operation of the pixel circuit 81 shown in FIG. 37. Prior to time point t91, the scanning line SL is in a non-selected state. Therefore, prior to time point t91, the transistor T1 is in an off state and the potential at the gate node VG maintains an initial level (e.g., a level determined depending on writing performed in the preceding frame). When reaching time point t91, the scanning line SL goes into a selected state and the transistor T1 goes into an on state. By this, a data voltage Vdata corresponding to the luminance of a pixel (subpixel) formed by the pixel circuit 81 is supplied to the gate node VG through the data line DL and the transistor T1. Thereafter, during a period up to time point t92, the potential at the gate node VG changes depending on the data voltage Vdata. At this time, the capacitor Cst is charged to a gate-source voltage Vgs which is the difference between the potential at the gate node VG and the source potential of the transistor T2. When reaching time point t92, the scanning line SL goes into a non-selected state. By this, the transistor T1 goes into an off state and the gate-source voltage Vgs held in the capacitor Cst is fixed. The transistor T2 supplies a drive current to the organic EL element OLED, depending on the gate-source voltage Vgs held in the capacitor Cst. As a result, the organic EL element OLED emits light at a luminance determined depending on the drive current.
Meanwhile, the organic EL display device typically adopts a thin-film transistor (TFT) as a drive transistor. However, the thin-film transistor is likely to cause variations in characteristics (threshold voltage and mobility). When variations occur in the characteristics of the drive transistors provided in the display unit, variations occur in the magnitude of a drive current. As a result, luminance nonuniformity occurs on a display screen, degrading display quality. In addition, regarding the organic EL element, the current efficiency (light emission efficiency) decreases with the passage of time. Therefore, even when a constant current is supplied to the organic EL element, the luminance gradually decreases with the passage of time. As a result, burn-in occurs.
Hence, conventionally, regarding the organic EL display device, there is proposed a technique for compensating for degradation of circuit elements such as drive transistors or organic EL elements. For example, WO 2014/021201 A discloses a configuration in which both the threshold voltage compensation and gain compensation of a drive transistor are performed for each pixel circuit.